1. Field of the Invention
This invention relates to an improvement in seal apparatus suitable for use in particle beam lithography systems.
2. Prior Art
U.S. Pat. No. 4,524,261 of Petric et al entitled "Localized Vacuum Processing Apparatus" (hereinafter called the '261 patent) and U.S. Pat. No. 4,528,451 of Petric et al entitled "Gap Control System For Localized Vacuum Processing" (hereinafter called the '451 patent).
The use and advantages of a so-called localized vacuum processing apparatus in a particle beam lithography system for treating a workpiece, such as a semiconductor wafer or mask, by a particle beam are disclosed in the '261 patent and the need for controlling of the gap length (size) between the tip of the localized vacuum processing apparatus and the workpiece are disclosed in the '451 patent.
The localized vacuum processing apparatus of the '261 patent comprises a pan-shaped housing member in which a lower plate and an upper plate member are nested so that centrally located apertured sleeves of each member are disposed concentrically. The pan-shaped housing member contains ports which are connected to vacuum pumps to create zones of radially inwardly increasing vacuum at the sleeve apertures with the centermost aperture in the housing member being at the highest vacuum. In this manner, only the area of the workpiece proximate the sleeves are subjected to vacuum while the rest of the workpiece surface is at ambient pressure. The gap between the tips of the sleeves and the workpiece surface is important and in the '451 patent there is disclosed a sensing and feedback control system for continuously monitoring and changing the gap length so as to be within a selected range by moving the workpiece up and down during workpiece processing. In the '261 patent, as well as in the '451 patent, the workpiece is moved transversely of the particle beams and of the localized vacuum processing apparatus to treat the workpiece in an otherwise conventional manner.
The localized vacuum processing apparatus of the '261 patent requires large vacuum pumps to evacuate relatively large areas because the ports are located only in the outer periphery of the outer housing member and because the gas channels to connect the ports to the sleeve apertures are formed by the nesting upper and lower plate members which extend the entire internal cavity area of the outer housing member. The amount of high vacuum obtainable was relatively low, on the order of 10.sup.-5 Torr, and the gap size was relatively large, on the order of not less than 20 microns.
This invention improves this prior art by providing a seal apparatus (also referred to as a differentially pumped seal apparatus) which has better pump efficiences by increasing gas channel capacities in the apparatus and therefore obtaining higher vacuum conductance, allows smaller capacity vacuum pumps to be used, provides a much smaller gap between the workpiece and the seal apparatus (in the order of 12 to 15 microns, + or -3 microns,) and is capable of a vacuum of 10.sup.-6 Torr which is a ten fold or better increase over the vacuum used in the prior art.